1. Field of the Invention
This invention relates to a new silacyclohexane compound, a method of preparing it, and a liquid crystal composition which contains it as well as a liquid crystal display element which contains said liquid crystal composition.
2. The Prior Art
A liquid crystal display element utilizes the optical anisotropy and dielectric anisotropy of liquid crystal substances. Display methods include the TN mode (twisted nematic mode), the STN mode (super twisted nematic mode), the SBE mode (super birefringence mode), the DS mode (dynamic scattering mode), the guest-host mode, the DAP mode ("deformation of aligned phase" mode) and the OMI mode (optical mode interference mode). The most common display device has a twisted nematic structure based on the Schadt-Helfrich mode.
Properties required of the liquid crystal substance used in these liquid crystal displays are somewhat different depending on the display mode. However, a wide liquid crystal temperature range and stability against moisture, air, light, heat, electric fields, etc. are commonly required in all display methods. Furthermore, it is desirable for the liquid crystal material to have a low viscosity, and also to have a short address time, low threshold voltage and high contrast in the cell(s).
Currently, there is no single compound which satisfies all these requirements. In practice, liquid crystal mixtures are obtained by mixing several to more than ten liquid crystal compounds and liquid crystal like compounds. Because of this, it is also important that components of a liquid crystal composition mix easily each other.
Liquid crystal compounds which can be used as these components can be classified into the following categories:
1) Compounds which contribute to a reduction in viscosity and a lowering of the melting point of the mixed liquid crystal composition PA1 2) Compounds which mainly control the electro-optical functions of the mixed liquid crystal composition PA1 3) Compounds which contribute to raising the clearing point of the mixed liquid crystal composition PA1 4) Compounds which contribute to refractive anisotropy of the mixed liquid crystal composition PA1 5) Compounds which control the colored display and orientation of the mixed liquid crystal composition PA1 comprising reacting an organometallic reagent: EQU R--M PA1 with a silacyclohexane compound: ##STR7## wherein Q denotes a halogen which is preferably Cl or Br, or alkoxy group. PA1 comprising reacting an organometallic reagent: ##STR9## wherein M' denotes MgP', ZnP' or Li, and P' denotes a halogen atom which is preferably Br or I when X is Cl and preferably Cl, Br or I when X is not Cl, PA1 with a silacyclohexane compound: ##STR10## wherein Q denotes a halogen which is preferably Cl or Br, or alkoxy group which is preferably an alkoxyl group with 1-4 carbon atoms. PA1 comprising reacting an organometallic reagent: ##STR12## wherein M" denotes MgP", ZnP" or Li, P" denotes a Br or I atom, and X' denotes a halogen atom which is preferably Cl or Br, with a silacyclohexane compound: ##STR13## wherein Q denotes a halogen which is preferably Cl or Br, or alkoxy group which is preferably an alkoxyl group with 1-4 carbon atoms, to obtain a phenylsilacyclohexane compound: ##STR14## forming a Grignard reagent: ##STR15## from said phenylsilacyclohexane compound by applying Mg thereto; and reacting a cyanation reagent: EQU T--CN PA1 Crystal.fwdarw.Nematic liquid crystal phase.fwdarw.Isotropic liquid PA1 Isotropic liquid.fwdarw.Nematic liquid crystal phase.fwdarw.Crystal PA1 1) A trans-1,4-cyclohexylene group which has no substitution or which has one or more substitutional groups such as F, Cl, Br, CN or alkyl groups, PA1 2) A ring comprising a cyclohexane ring in which O or S is substituted for one or nonadjacent two CH.sub.2 groups, PA1 3) A 1,4-cyclohexenylene group, PA1 4) A 1,4-phenylene group which has no substitution or which has one or two substitutional groups such as F, Cl, CH.sub.3 or CN groups, or PA1 5) A ring comprising a 1,4-phenylene group in which an N atom is substituted for one or two CH groups.
For compounds which belong to category 1) in this classification, compounds with so-called PCH structure such as ##STR3## have been known.
In recent years, along with the expansion of the applications of liquid crystal displays, the characteristics required of liquid crystal materials are becoming more and more advanced and demanding. In particular, superior characteristics such as improved low temperature performance, a wider temperature range for automobile onboard use and a lower driving voltage, compared with conventional liquid crystal substances, are desired.